82 VJLORA INDH1A.
The normal mean temperature of the equator is stated by
Dove to be a very little below 80°, but this is somewhat
exceeded in many parts of cbntinental India. The normal
mean temperature scarcely diminishes at all between 0° and
10° N. lat. Between 10° and'20° it diminishes 21°; between
20° and 30°, 7°; and between 30° and 40°, IS'3°. In 20° N.
lat. therefore the diminution may be estimated at about half
a degree of temperature, and in 30° N. lat. iit 1° of tempera-
ture, for a degree of latitude. In India, however, the mean
temperature does not diminish so rapidly., owing to the in-
crease of the mass of land to the northward, which, as has
been shown, becomes excessively heated in summer. fl*he
normal difference of temperature .between summer and winter
is least at the equator, and increases with the latitude; and
this effect is enhanced in India T)y the increase in the muss
of- land, which makes the summers hotter and the winters
colder than the average.
The phenomena of vegetation are less dependent upon the
moar temperature of the -year than upon that od' the season
of growth: thus, within the tropics, vegetation is active at
all periods of the year, but iu the cooler temperate zone, and
at considerable elevations on the mountain!* of the tropics,
only during the summer season. It is therefore important in
the investigation of climate with regard to its application to
botany, to know the mean temperature of each of the four
seasons, and, if possible, that of each month.
The only other important element,by which climate is af-
fected, is elevation above the level of the sea. The dimi-
nution of temperature as we ascend (on the surface of the
gcnem, and individual species aro extremely aen&itivo to the amount of mote-
iuro in, the air, and its fluctuations. Some plants arc confined to pcnnuiial hu-
midity, others to perennial drought, whilst Htill otlwnt aro tlejvncltmt <m UCCOB-
*ious of heat or drought at certain fixed periods, for Hfo and lumlth or tho
means of propagation. Comparatively few observations on temperature, and
those in certain months only, give us a sufficient approximation to tho re-
quirements of a plant iu |hat particular, but tin* hygronujtrical observation*
should be continued throughout tho year.